Hydrogen cyanide (HCN), HCN
CAS number: 74-90-8
Hydrogen cyanide is, depending on the physical conditions, a colourless gas, a liquid or a solid. It has a characteristic odour, which is often described as almond-like. Its solution in water forms a weak acid, commonly known as hydrocyanic or prussic acid. Hydrogen cyanide melts at 14 degrees Celsius and boils at 26 degrees Celsius. It is strongly flammable and it is a potentially explosive hazard when mixed with air.
Hydrogen cyanide is found in small quantities in the natural environment, especially in a variety of plants. Cherry seeds, peach and plum pits, corn, chickpeas, cashews, and some other fruits and vegetables contain cyanogenic (i.e., cyanide-forming) glycosides that release hydrogen cyanide when chewed or digested. Since the concentration of cyanide in these compounds is small, accidental cyanide poisoning from a food source is rare. However, most hydrogen cyanide is released into the environment through natural occurring processes, especially as biomass burning, and through industrial manufacturing activities such as electroplating, metallurgy, metal mining and metal cleaning processes. Another source of release is from cigarette smoking.
Relevant sources of emissions are the production and processing of metals, the chemical industry, coke ovens and combustion installations.
Hydrogen cyanide is very toxic by ingestion, skin contact and by inhalation. It inhibits oxygen binding respiratory enzymes and consequently the brain and the cardiovascular system are the main organs affected by hydrogen cyanide poisoning due to its high oxygen demand.
Hydrogen cyanide is generally not persistent when released to water and soil, and is not likely to accumulate in aquatic life. It can rapidly evaporate from water and soils and can be broken down by microbes. It is however very toxic to fish and other forms of aquatic life. Hydrogen cyanide acts as a respiratory poison to aerobic organisms, interfering with oxygen binding to respiratory enzymes. Hydrogen cyanide generally does not bind to soils and may leach to groundwater. At high concentrations, hydrogen cyanide is toxic to various soil micro-organisms.